Heat exchanger for a gas turbine engine

ABSTRACT

Solid ceramic plugs are cemented into axial grooves at the periphery of a ceramic regenerator core with a cement having a high coefficient of thermal expansion. The thermal expansion of the cement exerts forces on the rim portion of the regenerator core to counteract tensile stresses in the rim portion caused by expansion of the core interior. A foam cement having low thermal expansion can be used on the inner area of the plug and high expansion cement can be used only on the outer area.

United States Patent inventor Joab J. Blech Oak Park, Mich. Appl. No.855,175 Filed Sept. 4, 1969 Patented Mar. 9, 1971 Assignee Ford MotorCompany Dearborn, Mich.

HEAT ExcHANGER Eon A GAS TURBINE ENGINE 5 Claims, 2 Drawing Figs.

US. Cl 165/8, 165/10 Int. Cl F28d 19/00 Field of Search l65/8l0 [56]References Cited UNITED STATES PATENTS 3,401,741 9/1968 Palusznyetal.

Primary ExaminerAlbert W. Davis, Jr. Att0rneysJohn R. Faulkner and GlennS. Arendsen ABSTRACT: Solid ceramic plugs are cemented into axialgrooves at the periphery of a ceramic regenerator core with a cementhaving a high coefficient of thermal expansion. The thermal expansion ofthe cement exerts forces on the rim portion of the regenerator core tocounteract tensile stresses in the rim portion caused by expansion ofthe core interior. A foam cement having low thermal expansion can beused on the inner area of the plug and high expansion cement can be usedI only on the outer area.

PATENTED mm 9 Ian INVENTOR JOAB a. 54:?

ATTORNEYS HEAT EXCHANGER FOR A GAS TURBINE ENGINE SUMMARY OF THEINVENTION When the ceramic regenerator core for high temperature gasturbine engines was developed, a relatively new rim-type driving systemfor the core also was introduced. Serious cracking problems wereencountered in initial testing of the ceramic core and rimtype drivingsystems. Several different arrangements for assembling the driving ringgear to the core periphery to eliminate stresses have been proposed andtested, but none of the systems has been completely satisfactory ineliminating core cracking, especially when gas temperatures areincreased to increase engine efficiency.

This invention resulted from the discovery that core cracking was causedby thermally induced stresses in the core itself rather than by drivingstresses caused by the relationship of the driving gear tothe coreperiphery. Careful analysis showed that the interior portion of theregenerator core reached temperatures exceeding 1.3.0.0 F. while the rimportion of thecore, which was subjected to the relatively cool gasesfrom the engine compressor, generally operated at about 450 F.

The resulting thermal expansion of the interior portion of the coreproduces tensile stresses in the rim portion and these tensile stresseseventually result in cracks appearing at the rim. This inventioneliminates the cracks by forming a plurality of grooves in the outersurface of the rim of a regenerator and filling the grooves with amaterial having a high coefficient of thermal expansion. As theregenerator core warms up during engine operation, the material exertsexpansion forces on the rim portion that reduce the tensile stresses inthe rim below the'point where cracking occurs for all anticipatedoperating temperatures.

Solid ceramic plugs are used in regenerator cores utilizing the rimdrive techniques to transmit driving forces from the ring gearsurrounding the core to the core itself. These plugs can be bonded intothe core by a cement made up of high thermal expansion material. Ahighly useful arrangement involves bonding a portion of the outer areaof the plug to the regenerator core with the high thermal expansioncement and bonding the remaining portion of the plug bond area to thecore with a cement having a thermal expansion approximately equal to thethermal expansion characteristics of the core itself. In thisarrangement, the low expansion cement retains the plug in place despitecentrifugal, thermal, and drive loads while the high expansion materialexerts the compressive forces on the rim that prevent rim cracking.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of adisc-shaped ceramic regenerator core having a plurality of driving plugsbonded into its outer periphery.

FIG. 2 is a perspective view of a portion of the regenerator coreshowing a plug bonded to the core by the two types of ce ments havinghigh and low thermal expansion characteristics.

DETAILED DESCRIPTION A disc-type ceramic regenerator core is representedin the drawings by numeral 10. Regcnerator core has a solid ceramic hub12 surrounded by a gas conducting ceramic material 14. Material 14comprises a plurality of thin ceramic walls defining a plurality of gasconducting passages that conduct gases in a substantially axialdirection through the regenerator core. Porous material 14 extendscompletely out to the outer periphery of the regenerator core. Whenmounted in the engine, one face 15 of the core is exposed to hot gasesleaving the engine turbine wheels and this face along with its axiallyadjacent parts of the core operate at a higher temperature than theremainder of the core.

A plurality of grooves 16 are spaced along the outer periphery of theregenerator core. Grooves 16 generally parallel the rotational axis ofthe core. Mounted in each groove 16 is a solid ceramic plug 18. Bothgrooves 16 and plugs 1% have a cross section substantially correspondingto a sector of a cylinder.

Plugs 18 are bonded into grooves 16 with two types of cements 20 and 22.Cement 20 is a foam type inorganic cement having thermal expansionproperties substantially equaling the thermal expansion properties ofporous material 14. A typical foam cement useful in this invention canbe obtained from the Dow Corning Corp. Foam cement 20 is applied to theinner sector 21 of the cylindrical surface of each plug for a shortdistance from the hot face 15 and to the entire bond surface for theremainder of the plug.

Cement 22 is a material having thermal expansion props exceeding thermalexpansion properties of material 14. A typical high expansion cementcomprises mullite in sodium silicate and can be obtained from theCarborundum Company. Cement 22 is applied to the outer sectors of eachplug on both sides of inner sector 21. Plugs 18 can be attached to anannular ring gear to transmit driving forces to the regenerator core inany desired manner '(See, for example, U.S. Pat. No. 3,401 ,74l Palusznyet al.

In a typical disc-type regenerator having a diameter of approximately 28inches and an axial thickness of about 4 inches, plugs 18 aresemicylindricaland have a radius of approximately 1 inch. Cements 20 and22, are applied in layers that produce a thickness of about 0.1 inchafter curing. Cement 22 typically is applied to at least the radiallyouter 0.5 inch for an axial distance ,of about 12 inches of the surfacesbetween the regenerator core and the plugs.

During engine operation, hot combustion gases passing through theregenerator heat the central portion. of face 15 and its axiallyadjacent portions of the regenerator to temperatures exceeding l,300 F.The periphery of the regenerator is subjected to the relatively cool airleaving the engine compressor and operates at about 450 F. As thetemperature of cement 22 rises to its operating point, the high thermalexpansion properties thereof exert compressive forces in the directionof arrows 24 the material 14 in the rim portion of the regenerator core.These compressive forces reduce the tensile stresses in the rim portionand thereby reduce the cracking tendencies of the core. Cement 20 doesnot expand to the same degree as cement 22 and provides strong adhesionbetween the core and the plug to hold the plug in place.

If desired, cement 22 can be applied to the entire axial thickness ofcore 10 on both sides of a sector of cement 20. in some cases, cement 22can be used exclusively. Grooves 16 can be reduced considerably in sizeand filled only with a cement having the high thermal expansionproperties of cement 22. The number of grooves generally dependson theexpansion characteristics of the core and the cement and the anticipatedoperating conditions of the engine. A few empirical tests generally aresufiicient to determine both the number and size of the grooves. Thinslats of a material having a high thermal expansion can be located inthe grooves. Alternatively, the grooves can be shaped like scallopedslots extending for short distances into the core fromhot face 15 at thecore I periphery and the slots filled with high expansion material.

The high expansion cement preferably has a higher coefficient of thermalexpansion for temperatures ranging up to its operating temperature,usually about 450 F., and a reduced coefficient of expansion attemperatures between the operating temperature and the higher firingtemperature of the two cements, which usually is about l,000 F. A graphof the expansion characteristics of the two cements thus shows theexpansion curve of the high expansion cement rising rapidly totemperatures of about 450 F. and then leveling off to approach or crossthe expansion curve of the other cement. The mullite and sodium silicatecement disclosed above has these characteristics. This arrangementpermits firing the core and plug assembly without inducing excessivestresses in the bonds of the plugs to the core.

Thus this invention provides a heat exchanger for a gas turbine enginethat has improved physical integrity and a long service life even athigh operating temperatures. Heat exchangers having a rim type drivingsystem can incorporate the invention with a minimum of expense.

lclaim:

1. In a gas turbine engine, a rotary heat exchanger comprising aregenerator core made of a ceramic material, said core having aplurality of thin walls defining a plurality of gas-conducting passages,said core having a central portion of at least one face subjected torelatively high temperatures and a rim portion surrounding that centralportion, said rim portion being subjected to lower temperatures, saidregenerator having a plurality of grooves in the outer surface of saidrim portion, and a second material having a high coefficient of thermalexpansion relative to the ceramic material of the core in said grooves,said second material exerting expansion forces on the rim portion toreduce tensile stresses in said rim portion.

2. The engine of claim 1 comprising a solid ceramic plug located in eachof said grooves, said plugs being bonded in place by a cement made up ofsaid high thermal expansion material.

3. The engine of claim 2 in which the regenerator core is disc-shapedand the gas-conducting passages run substantially axially through saiddisc, said grooves running axially along the outer periphery of the coreand having a cross section corresponding to a sector of a cylinder, saidplugs having a cross section substantially corresponding to the crosssection of said grooves, the inner area of the surface of each plugbeing bonded to the core with a cement having a thermal expansionapproximately equal to the thermal expansion of the core, and the outerarea being bonded to the core by said high thermal expansion cement.

4. The engine of claim 3 in which the high expansion cement is used onlyin said outer areas of the surface between the plug and the core at thehot face of the core and for a short axial distance from said hot faceof the core.

5. The engine of claim 4 in which the high expansion cement has acoefficient of expansion greater than the core at temperatures up to itsnormal operating temperature and a reduced coefficient of expansion athigher temperatures.

1. In a gas turbine engine, a rotary heat exchanger comprising aregenerator core made of a ceramic material, said core having aplurality of thin walls defining a plurality of gas-conducting passages,said core having a central portion of at least one face subjected torelatively high temperatures and a rim portion surrounding that centralportion, said rim portion being subjected to lower temperatures, saidregenerator having a plurality of grooves in the outer surface of saidrim portion, and a second material having a high coefficient of thermalexpansion relative to the ceramic material of the core in said grooves,said second material exerting expansion forces on the rim portion toreduce tensile stresses in said rim portion.
 2. The engine of claim 1comprising a solid ceramic plug located in each of said grooves, saidplugs being bonded in place by a cement made up of said high thermalexpansion material.
 3. The engine of claim 2 in which the regeneratorcore is disc-shaped and the gas-conducting passages run substantiallyaxially through said disc, said grooves running axially along the outerperiphery of the core and having a cross section corresponding to asector of a cylinder, said plugs having a cross section substantiallycorresponding to the cross section of said grooves, the inner area ofthe surface of each plug being bonded to the core with a cement having athermal expansion approximately equal to the thermal expansion of thecore, and the outer area being bonded to the core by said high thermalexpansion cement.
 4. The engine of claim 3 in which the high expansioncement is used only in said outer areas of the surface between the plugand the core at the hot face of the core and for a short axial distancefrom said hot face of the core.
 5. The engine of claim 4 in which thehigh expansion cement has a coefficient of expansion greater than thecore at temperatures up to its normal operating temperature and areduced coefficient of expansion at higher temperatures.